There have been several mass shooters who have taken the lives of innocent people in the past several years and unfortunately, the news of each new incident begins to sound familiar.

The most recent multiple shooting occurred Tuesday on the campus of YouTube’s headquarters located in San Bruno, California but there was a major difference from other shootings that have recently occurred in the country: the incident involved a female shooter.

Nasim Aghdam, a 39-year-old San Diego resident, shot and wounded four people at YouTube headquarters Tuesday. She was allegedly found dead from a self-inflicted gunshot wound.

The FBI defines mass shooting as an incident involving “four or more people shot at once.” It is still unclear if this recent incident is to be considered a mass shooting, the fact that it was a female who was identified as the shooter is not common when looking at mass shootings in the country statistically.

Female mass shooters

According to data from a 2016 FBI study a total of nine of the 220 incidents the FBI considered as mass shootings involved female shooters. The study looked at data from active shooter incidents in the United States from 2000 to 2016. In 2017 there were 28 mass attacks in the U.S. and there were no female perpetrators in any of the incidents, according to the U.S. Secret Service.

There has been extensive research about why mass shooters are rarely female. Some research and theories suggest that men are naturally more aggressive than women, a characteristic which may contribute to people becoming mass shooters.

Candice Batton, director of the School of Criminology and Criminal Justice at the University of Nebraska, Omaha, told NPR in 2013 that “some research supports the idea that males are more likely than females to develop negative attributions of blame that are external in nature, that is: ‘The cause … of my problems is someone else or some force outside of me.’ And this translates into anger and hostility toward others.”

According to Batton, women "are more likely to develop negative attributions of blame that are internal in nature, that is: 'The cause of my problems is some failing of my own: I didn't try hard enough, I'm not good enough.' And this, in turn, tends to translate into feelings of guilt and depression that are targeted toward oneself."

Her comments on the topic of mass shootings were made in the wake of the Washington Navy Yard shooting, where 34-year-old Aaron Alexis shot and killed 12 people.